The present invention relates to a DLL (Delay Locked Loop) circuit and, more particularly, to a DLL circuit having the function of reducing intensity of electromagnetic radiation noise.
An LSI mounted on an electronic device emits electromagnetic waves based on power consumption and operation frequency. Electromagnetic waves exert an adverse influence such as electromagnetic radiation noise on an entire electronic device. The main cause of the electromagnetic radiation noise is electromagnetic waves generated from a DLL (Delay Locked Loop) circuit provided for the LSI and other circuits using an output signal (DLL output signal) of the DLL circuit. In particular, strong electromagnetic waves are generated from the other circuits using the DLL output signal.
On the other hand, as electronic devices are sophisticated and the integration technique has improved in recent years, the power consumption, operation frequency, packaging density, and the number of LSIs mounted on an electronic device have been increasing. As a result, the intensity of electromagnetic waves (electromagnetic radiation intensity) emitted from the LSI increases, and the influence of electromagnetic radiation noise exerted on the electronic device is becoming unignorable.
A ordinary LSI has a DLL circuit. When the DLL circuit is in a locked state (that is, the DLL circuit and the LSI having the DLL circuit are in an operating state), the frequency of a DLL output signal of the DLL circuit has a constant value (DLL locked frequency). As a result, the electromagnetic radiation intensity at the DLL lock frequency becomes very high.
A DLL circuit suppressing minimum delay time (minimum slew rate) of delay time which can be adjusted in a wide range is known (Japanese Patent Application Laid-open Publication No. 2004-260663). However, the Japanese Patent Application Laid-open Publication No. 2004-260663 does not disclose means for reducing the electromagnetic radiation noise caused by the DLL circuit.
Therefore, the electromagnetic radiation intensity of the conventional DLL circuit and that of an LSI having the DLL circuit is high. There is a problem such that an electronic device having the LSI is adversely influenced by the electromagnetic radiation noise.